1. Field of the Invention
This invention relates generally to marine seismic data acquisition, and more particularly to the compensation of seismic receiver motion during such data acquisition.
2. Description of the Related Art
Seismic surveying is utilized to define subsurface geology and thereby improve the likelihood of successfully drilling into a subsurface formation that contains recoverable hydrocarbons. Surveys may be taken over land or sea, and typically involve the generation of a seismic wave via an explosion, impact, or vibration triggered at the surface by a seismic source. The structure of a subsurface formation is mapped by measuring the times required for the seismic wave to return to a plurality of receivers at the surface after having been reflected from interfaces between various subsurface formations, also known as sediment layers, having different physical properties. Variations in the reflection times from one receiver to another on the surface generally indicate structural features in the strata below the receivers.
Once seismic energy waves begin propagating from a seismic source, the waves are independent of the source and become dependent only on the medium through which the waves are conveyed. This medium is effectively stationary, whether the seismic operation is on land or at sea. Thus, seismic energy waves generated in a marine operation propagate through a stationary medium and are then recorded by a moving receiver. This produces a distortion in the recorded seismic data. The distortion is illustrated by considering a streamer cable carrying seismic receivers behind a marine vessel moving at 2.25 m/sec. A seismic wave reflection arriving 4 seconds after the xe2x80x9cshot xe2x80x9d from the seismic source will be recorded by seismic receivers that have moved 9 meters away from their location at the time of the shot. Since the nominal receiver location would be their location at the time of the shot, an error of 9 meters in offset and 4.5 meters in midpoint is introduced by the receiver motion.
Source motion has long been recognized as a Doppler shift problem for marine vibrators, but has generally been regarded as negligible for impulsive sources such as marine air guns. However, it is now realized that the effects of receiver motion cannot be neglected. The receiver motion is the same regardless of the source, and is more significant with 3D and 4D data.
To address this problem, it is a principal object of the present invention to provide a time-variant static correction to compensate for the effects of receiver motion in marine seismic surveying.
It is a further object that the correction be offset-dependent.
The objects described above, as well as various other objects and advantages, are achieved by a method of compensating for seismic receiver motion in a marine seismic survey wherein at least one receiver is towed behind a seismic vessel. The method comprises producing an acoustic energy wave at a seismic source, and recording the reflection arrival time t2 of the acoustic energy wave at the one receiver. The offset x between the source and the one receiver is determined. The normal moveout velocity V for the one receiver relative to the source, and the velocity VB of the seismic vessel, are also determined. A corrected reflection arrival time t1 of the acoustic energy wave is then determined by applying a time correction to the recorded reflection arrival time t2. The time correction is a function of the offset x, the normal moveout velocity V, and the velocity VB of the seismic vessel.
In a particular embodiment of the present invention, the one receiver is towed by the seismic vessel towards the source, and the corrected reflection arrival time t1 is determined according to the equation:
t1=t2+x(VB+V2).
In another embodiment of the present invention, the one receiver is towed by the seismic vessel away from the source, and the corrected reflection arrival time t1 is determined according to the equation:
t1=t2xe2x88x92x(VB/V2).
In a typical application of the present invention, the source is an air gun or an array of air guns and the one receiver is a hydrophone.
The method is applicable to a plurality of receivers, whether the receivers are arranged in a single sensor acquisition system or hardwired to form receiver groups.